1. Field of the Invention
The present invention relates to a thermal type infrared sensing device, a method of manufacturing the thermal type sensing device, and an infrared imaging system and infrared imaging apparatus.
2. Related Art of the Invention
In recent years, the need has been growing for security monitoring and air conditioning control involving detecting the presence or absence of humans in a room and the amount of their activity With this trend, apparatuses for detecting infrared radiation sources by using infrared sensors have come into use in order to detect a human body by detecting the infrared radiation emitted from the human body and to control environment control equipment, such as air conditioners and lighting equipment, security systems, or the like, by using the obtained signal. Furthermore, as the need increases to detect undesirable events by monitoring the temperature distribution of an object such as a heat source in real time, the demand for two-dimensional infrared sensing devices is increasing. It is therefore desired to develop a low-cost, high-performance two-dimensional infrared sensing device.
Two types of infrared sensors are known: quantum type sensors that detect infrared radiation as photons, and thermal type sensors that utilize a change in the physical properties of the device when the temperature of the device rises by absorbed infrared radiation. Since quantum type sensors usually need cooling by liquid nitrogen or the like, thermal type sensors are commonly used. Of the thermal type sensors, the pyroelectric infrared sensor is suitable for detecting an infrared radiation source because of its high sensitivity compared with other types of thermal type sensors; however, since the pyroelectric infrared sensor is basically intended for detecting changes in infrared radiation, if it is to be used for the detection of a stationary infrared radiation source, provisions must be made by some method so that the infrared light is incident intermittently on the light sensitive area of the sensor. Usually, the intermittent interruption (chopping) of the infrared light is accomplished by rotating a chopper constructed with a slitted disk or plate.
Thermopile type, which detects a thermal electromotive force developed between metals, is another type of thermal type infrared sensor. Since the thermopile type sensor utilizes a thermal electromotive force generated by a temperature difference between hot and cold junctions, the device construction is large. Bolometer type is one that detects a change in resistivity, but with this type of sensor, the rate of change of resistivity is not sufficiently large. Dielectric bolometer type is also one type of thermal type infrared sensor. This type of sensor detects the change in permittivity in relation to the temperature change but is not yet ready for practical use. These types of sensors do not need a chopper but need the application of a voltage.
FIG. 17 is a schematic diagram showing a cross sectional structure of a pyroelectric element in a prior art pyroelectric infrared sensor. In the illustrated pyroelectric infrared sensor, light-receiving electrodes 162 for receiving infrared radiation and compensation electrodes 163, one for each of the light-receiving electrodes 162, are formed on the upper surface of a dielectric film 161 which also serves as the substrate, and first counter electrodes 164 and second counter electrodes 165 are formed on the lower surface of the dielectric film 161 in such a manner as to oppose the light-receiving electrodes 162 and compensation electrodes 163, respectively, formed on the upper surface. Output connection patterns 166 and 167 are connected to the light-receiving electrodes 162 and compensation electrodes 163, respectively. The first counter electrode 164 and second counter electrode 165 opposing one pair of light-receiving and compensation electrodes 162 and 163 are electrically interconnected, though not shown explicitly in the figure. With infrared radiation falling only on the light-receiving electrode 162 on the upper surface of the dielectric film 161, a potential difference occurs, and by detecting the resulting voltage, an infrared radiation source can be detected. In this arrangement, since the potential difference is relative to the compensation electrode, variations between sensing elements can be reduced, but cannot be eliminated completely, and sensitivity variations of about 10% occur.
On the other hand, with sensors that do not have compensation electrodes such as described above, since the characteristics of the dielectric film are reflected directly in the output, large variations in sensitivity can occur. In some sensors, corrections are done in software.
Thermal type infrared sensors that detect infrared radiation as described above are capable of detecting a heat radiation source by examining the temperature distribution in a space to be measured. In the prior art sensor, which, for example, is configured to form eight independent detection zones using eight infrared light receiving electrodes 162, when infrared radiation from a human body as an infrared radiation source is incident only on one receiving electrode 162, normally an output signal should be produced only from that one light-receiving electrode 162. As it is, however, the heat of the infrared radiation received by that one infrared receiving electrode 162 is conducted through the dielectric film 161 to other infrared light receiving electrodes 162, causing a temperature rise in those other electrodes and producing the same polarization as if infrared radiation were received by them; as a result, a potential difference also occurs here and is output as an output signal. The problem of thermal crosstalk thus occurs.
This thermal crosstalk, causing other electrodes to produce output signals by heat conduction when infrared radiation is not incident on them, increases an apparent output, blurs the infrared image, and leads to erroneously judging that the heat radiation source is larger than it actually is. The resulting problem is that the position of the heat radiation source cannot be detected accurately. The above description has dealt with the thermal crosstalk between the light-receiving electrodes, but thermal crosstalk from the light-receiving electrode to its associated compensation electrode can likewise occur, causing the problem of degraded reliability of the compensation electrode which should normally work to compensate the output of the receiving electrode without being affected by the infrared radiation.
To suppress the crosstalk, and to prevent infrared light from falling upon the compensation electrode, one possible approach would be to dispose each compensation electrode sufficiently spaced apart from its associated light-receiving electrode, but this would in turn present a problem in terms of device size reduction. A further problem is that, since the compensation electrodes are arranged alongside their associated light-receiving electrodes, the device further increases in size and the construction does not lend itself to device miniaturization and two-dimensional device design.
That is, the whole problem of the thermal crosstalk, which involves heat conduction between the light-receiving electrodes or between the light-receiving and compensation electrodes, is that when infrared radiation is incident on a given light-receiving electrode, outputs are also produced from its neighboring electrodes, causing variations and errors in the output of space sensing and making it impossible to accurately detect the heat radiating object. A further problem is that if the crosstalk is to be suppressed sufficiently, the device size necessarily increases, posing a barrier to achieving higher resolution and device miniaturization.
Furthermore, with the provision of the compensation electrodes, since the sensor output can be detected as the potential difference between a light-receiving electrode and its associated compensation electrode, variations in the characteristics of the dielectric film itself can be offset to some degree, and sensitivity variations between sensing elements can be suppressed to a certain extent; however, there remains the problem that the sensitivity variations cannot be suppressed sufficiently and sensitivity variations of 10% or greater occur.
On the other hand, sensors without compensation electrodes have the problem that large sensitivity variations can occur between sensing elements because the characteristics of the dielectric film are reflected directly in the output. In some sensors, corrections to sensitivity are done in software on a pixel by pixel basis, but the problem is that this involves an extremely complicated process.
More specifically, the problem is that as the number of infrared sensing elements increases, the difference in the characteristics of the dielectric film becomes more pronounced, and sensitivity variations between sensing elements become very large, causing variations and errors in the output of space sensing and making it impossible to accurately detect the heat radiating object. If the variations are to be suppressed, then complicated procedures using software become necessary, posing a barrier to achieving higher resolution and lower cost construction.
In view of the above-outlined problems with the prior art thermal type infrared sensing devices, it is an object of the present invention to provide a low-cost thermal type infrared sensing device and a fabrication method for the same, wherein the device size is reduced to achieve the miniaturization and the two-dimensional, high-resolution design of the device construction. It is also an object of the present invention to provide a low-cost thermal type infrared sensing device and a fabrication method for the same, wherein in addition to reducing the device size, the light-receiving electrodes and compensation electrodes are formed in such a manner as to suppress thermal crosstalk, thereby preventing the blurring of infrared images and achieving the miniaturization and the two-dimensional, high-resolution design of the device construction. It is a further object of the present invention to provide an infrared imaging system that can suppress sensitivity variations among light-receiving electrodes, is capable of accurate and substantially error-free sensing of a space to be measured, and achieves high resolution. It is a still further object of the present invention to provide an infrared imaging apparatus equipped with the above thermal type infrared sensing device or infrared imaging system.
A thermal type infrared sensing device of the present invention comprises: a plurality of light-receiving electrodes for outputting a change of surface charge associated with a polarization that occurs in a dielectric when subjected to infrared radiation; and a single compensation electrode for compensating the output of each of said light-receiving electrodes.
A method of the present invention for fabricating a thermal type infrared sensing device, comprises the steps of: forming a compensation electrode on a substrate; forming a second dielectric member on top of said compensation electrode; forming a second counter electrode on top of said second dielectric member; forming a first counter electrode on top of said second counter electrode; forming a first dielectric member on top of said first counter electrode; and forming a light-receiving electrode on top of said first dielectric member.